This invention relates generally to analytical techniques. More specifically, it is directed to methods for determining the levels of acids or bases in various compositions, e.g., stripping solutions.
Acid and basic solutions have many uses, e.g., degreasing, electroplating, semiconductor metallization, cleaning, bright-dipping, etching, catalysis, and the like. They are often used as stripping compositions, e.g., to strip a metal coating from a metallic or non-metallic substrate. As a specific example, aluminide and platinum aluminide coatings used to protect superalloy components sometimes need to be stripped from the superalloy surface, e.g., during coating repair. (Aluminide coatings are often xe2x80x9cdiffusion coatingsxe2x80x9d, i.e., the aluminide material is diffused into the substrate surface).
Stripping compositions for aluminide coatings are often based on various acids or combinations of acids, e.g., hydrochloric acid, nitric acid, and phosphoric acid. Examples of these compositions are found in application Ser. No. 09/303,755 (D. Sangeeta et al), filed on May 3, 1999, and assigned to the assignee of the present invention. The compositions usually must exhibit a number of attributes, e.g., the ability to quickly remove the coating from the substrate; and to be effective at relatively low use temperatures. Usually, the compositions must also be capable of stripping the particular coating material (e.g., aluminide or platinum-aluminide), without attacking the base metal of the substrate. It should also be noted that the compositions sometimes must be selective in their ability to remove different coating materials.
Some of the very effective stripping compositions for aluminide-type materials contain specific mixtures of acids, along with various other ingredients. For example, one such composition contains methanesulfonic acid, hydrochloric acid, sodium hypophosphite, and water. Such a composition possesses the various attributes discussed above, e.g., removal of the desired coating without damaging the base metal.
Maximum effectiveness for such a composition is usually achieved when the components are maintained at fairly specific levels. If the stripping composition deviates from those levels, serious problems could occurxe2x80x94especially in a commercial, production-type setting. For example, the composition may not sufficiently remove all of the coating material. In some instances, use of the composition may remove the coating material, but may also cause pitting of the substrate surface. As a consequence, the time and expense involved with high scrap levels and process inefficiency can have severe economic ramifications.
Obviously, a composition can initially be prepared according to a desired formulation. However, the content of such a composition (e.g., a stripping bath) usually changes during its operation. For example, various bath components are consumed at different rates. Thus, monitoring the level of each component at regular intervals is critical to maintaining the composition within selected parameters.
It is a relatively easy task to determine the level of components in a xe2x80x9csimplexe2x80x9d composition, e.g., a stripping bath containing only one acid. For example, a titration technique can efficiently be used to determine the amount of acid present in the bath at any given time. If the amount is out of specification, the bath contents can readily be adjusted, e.g., by addition of more acid, dilution, addition of compensating base, and the like.
However, analysis is much more difficult in the case of a complex acidic composition, e.g., one containing combinations of acids. (The same is true for a complex caustic composition, as described below). Titration techniques will usually measure only the total acid level. Such a measurement is insufficient by itself, since an operator would not know which particular acid is out of specification. As alluded to earlier, the presence of each acid (and possibly the non-acid components) at a particular level is necessary to ensure selectivity in terms of what is removed and what is not removed from the substrate.
Therefore, new methods for analyzing the level of components in complex compositions would be welcome in the art. The methods should be capable of accurately measuring the quantity of each significant component in the composition. The methods should also be readily adaptable to a production system, e.g., a large-scale stripping operation for removing aluminide-type coatings. It would be very desirable if the new analytical techniques could be carried out without an excessive amount of additional expense, in terms of costly equipment and time-consuming procedures.
A primary embodiment of the present invention embraces a method for determining the concentration of at least one acid or base in a complex composition. As used herein, the term xe2x80x9ccomplex compositionxe2x80x9d is meant to embrace compositions which contain at least one acid or at least one base, in addition to at least one other component, other than the solvent-medium (e.g., water). The compositions are usually xe2x80x9csolutionsxe2x80x9d, i.e., uniformly dispersed mixtures, and that term is sometimes used herein. As further described below, the complex solutions analyzed according to the present invention are often acid mixtures, i.e., including more than one acid. They can alternatively be caustic mixtures, which include one or more bases with other components. The method comprises the following steps:
(a) measuring at least a first physical property value for the solution and a second physical property value for the solution, wherein the first physical property and the second physical property can be respectively defined in the form of a first solution property equation and a second solution property equation, wherein each equation expresses the respective physical property as a function of the concentration of one of the acids or bases;
(b) inserting the measured physical property values into the respective solution property equations, wherein the equations each include at least two unknown quantities, one of which is the desired concentration of the acid or base (i.e., the concentration being determined by this method); and
(c) solving the solution property equations simultaneously, so as to determine the desired concentration.
The physical properties are characteristic of the solutions being measured. They are usually selected from the group consisting of electrical conductivity, opacity, density, refractive index, electromagnetic wave transmission, electromagnetic wave absorption, fluidity, open potential of solution, sound velocity, and color.
The solution property equations are derived from measurements of physical properties in each solution. The property equations are then used to determine the concentration of the various components in the solutions. In preferred embodiments, each solution property equation is derived from a process comprising the following steps:
(I) preparing at least two standard compositions, designated as a first standard composition and a second standard composition, each of which contains components present in the solution, in selected amounts, wherein the concentration of each component in one of the standard compositions varies from that of the other standard composition, within a selected tolerance range;
(II) measuring first and second physical properties for each standard composition, to obtain first and second property values, respectively;
(III) incorporating a set of all first property values and their corresponding, varying concentrations into a regression analysis algorithm, to obtain the solution property equation for the first physical property; and incorporating a set of all second property values and their corresponding, varying concentrations into the regression analysis algorithm, to obtain the solution property equation for the second physical property. (The regression analysis is often performed with the aid of a computer). The selected tolerance range for each component is usually less than about 20% of the concentration of the component in the first standard composition. As described below, more than two standard compositions are often prepared, and designated as additional standard compositions.
The solution being analyzed can comprise a variety of acid or basic compositions. The acidic compositions often include one or more components such as sulfonic acid, hydrochloric acid, and phosphoric acid. The basic compositions often include water and a hydroxide base, such as sodium hydroxide.
As mentioned above and further described below, the solution is often a stripping composition for removing a coating from a metal substrate. For example, it can be used to remove an aluminide-based coating or a ceramic coating from the surfaces of a turbine engine component, e.g., one formed of a superalloy material.
The solution being analyzed according to this invention is often contained in an apparatus for removing coatings from metal-based substrates. As described below, the apparatus usually comprises a pressure vessel capable of containing the solution and the substrates while the substrates are treated in the solution; a storage tank for the solution; and at least one passageway between the storage tank and the pressure vessel, for carrying the solution therebetween.
Other details regarding the various embodiments of this invention are provided below.